The invention relates to a surveillance system for terrestrial navigational and airport landing systems.
Such navigational and landing systems are used in civil and military air travel, both for position and course fixing and for helping aeroplanes to land. Examples of navigational systems for position and course fixing include so-called VOR (Very High Frequency Omnidirectional Range) and DVOR (Doppler VOR) rotating radio beacons, which are predominantly employed in civil applications, and the TACAN system which is predominantly used in military applications. Also included in this category are distance measuring installations such as DME (Distance Measuring Equipment), and beacon systems such as NDMs (Non-Directional Beacons).
Examples of landing systems include the widely used instrument landing system ILS and the comparatively new microwave landing system MLS. Components in landing systems further include systems based on differential GPS (correction data transmission).
All the systems cited above entail installing transmitting stations on the ground which transmit corresponding signals; the signals are received on board the aeroplanes, evaluated in the navigational equipment for position and course fixing, and utilised in the landing aid facilities for determining localisation and angle of approach.
International authorities such as the ICAO standardise certain operating parameters throughout the world.
Safety aspects naturally play a defining role in determining these operating parameters, i.e. particularly in the case of the transmitting stations it is necessary to adopt measures that guarantee the maximum degree of safety both with regard to the construction of the transmitting stations and with regard to the operation thereof.
Such safety-related measures are laid down by the aforementioned ICAO international authority in the so-called International Standards and Recommended Practices. Where the integrity of the transmitted signals is at issue, then for example certain surveillance systems are provided such as the known Far Field Monitor (FFM) for the aforesaid ILS instrument landing system. This FFM (far field monitor) monitors the technical quality of the localising information in the final phase of the landing approach.
This type of known surveillance system principally monitors, and if necessary corrects, those faults that are contained in the transmitted signal itself.
In addition to such faults or signal distortions that occur within the system itself, other influences coming from outside also appear which can result in the signals here designated overall as navigational signals either being unable to be evaluated in the aeroplane or elsexe2x80x94and this may have more serious consequencesxe2x80x94leading to false evaluation results. Possible sources of such external faults may for example be radio transmitters, or the deliberate broadcasting of differential GPS data in the VHF band, or transmitters of other radio-communication services, which for example due to their own operating malfunctions broadcast interference signals in frequency ranges that are within range of the operating frequency of the navigational or landing system.
However, because as a rule such transmitters are fixed installations, the fact that they broadcast publicly means that their operating data are known and are readily identifiable, and systematic technical and organisational rectification of the problem is possible.
Far more dangerous faults or falsifications of the navigational signals may be caused by transmitters that are only introduced periodically into the geographical vicinity of the navigational or airport landing systems and operated there. For present purposes it is immaterial whether the perturbation or falsification of the navigational signal is due to a malfunction or to an intentional transmission.
Existing known surveillance systems for navigational and airport landing systems do not provide for systematic monitoring of such perturbations and are therefore not suitable either.
The patent U.S. Pat. No. 4,333,081 describes a monitoring system for scanning-beam microwave landing apparatus. The patent U.S. Pat. No. 3,818,476 describes a navigation aid transmitter-monitor system.
It is therefore the object of the present invention to create a surveillance system for terrestrial navigational and airport landing systems in which the navigational signals for aeroplanes or other airborne objects transmitted by the navigational and airport landing systems are received and evaluated by a ground-based receiving and control facility; it should also be possible to use said surveillance system effectively when faults or falsifications of the last-named type occur.
The measures cited in claim 1 are proposed in order to accomplish this object The advantage of this solution lies in the fact that the additional receiving stations and their geographical distribution enable position-fixing of the rogue transmitter using per se known methods, such as for example the hyperbolic sectioning method or the like, and the central evaluating unit, which in any case operates with conventional digital signal processing methods for the FFM mentioned in the introduction, such as for example FFT (Fast Fourier Transform) algorithms, is in addition able to identify the rogue signals.
For maximum simplicity and reliability of the co-operation between the additional receiving stations and the central evaluating unit, one embodiment of the invention provides for proceeding in accordance with the features of claim 2. Because the transmitting data of the system""s own transmitters and any stationary rogue transmitters are known in the central evaluating unit, it is possible in accordance with the features of claim 3 to ascertain the geographical co-ordinates of the unknown rogue transmitter. Claim 4 sets out how the content of the transmitting signal from the unknown rogue transmitter is evaluated.
According to a further embodiment of the invention corresponding to claim 5, it is advantageous that with central evaluation there is the possibility of selecting suitable counter-measures by comparing the navigational signal and the rogue signal.